Flash memory is a commonly used type of non-volatile memory in widespread use as mass storage for consumer electronics, such as digital cameras and portable digital music players for example. As process technology shrinks the cell size, an important issue for this type of memory is density of the memory. In order to achieve higher cell efficiency from the cell array, the peripheral block size may be optimized. In this regard, a challenge is an apparent need for power supplies having diverse voltage generators. As will be appreciated by those skilled in the art, the power supply generators of flash memories make different voltage levels according to the operating state of the memory. Also, generating a higher supply voltage from the source voltage requires pumping circuits to boost up to the high supply voltage from the source voltage, which is typically referred to as VDD. In some examples, VDD is about 1.5V, or even significantly less.
One existing method of generating more than one higher voltage levels from a single power supply voltage is to use a charge pump circuit having a fixed number of stages to generate a voltage level. Other voltage levels can be provided using voltage divider networks in conjunction with the single charge pump. A disadvantage of this approach is that power is wasted on the voltage divider networks. Another disadvantage may be that using a fixed number of stages sacrifices performance at one power supply voltage to accommodate another power supply voltage. Along with the above-mentioned disadvantages, each power supply voltage has different current driving capability based on the operating state and, as such, those skilled in the art will appreciate that a divider-based, power supplies generation approach may not be suitable in the context of flash memory.
Another approach in providing for two or more power supply generations is through the use of more than one electrical switches among more than one pumping circuits as shown in FIG. 1, a diagram of a charge pump 100 having a plurality of stages 104-107. It may be seen that the main purpose of the illustrated approach is to take two or more voltage levels from a same output 110. By controlling which of switches 112 and 114 are turned on, Vout voltage level can be varied depending upon whether voltage boosting from the pump stage pair 104, 105 is added to voltage boosting from the pump stage pair 106, 107 by series coupling of the pairs. Output current can also be varied by means of the switches 112 and 114. In particular, if both the switches 112 and 114 are turned off, only the pump stage pair 104, 105 sources output current through the output 110. If the switch 112 is off and the switch 114 is on, there is parallel coupling of the pump stage pairs, and both pump stage pairs source output current through the output 110.
Consider for a moment attempting to apply the approach of FIG. 1 in a non-volatile memory device such as, for example, a flash memory device. A problem that would be encountered is that if the Vout at the output 110 were to be used to make two or more power supply levels in the flash memory, there would be a common connection between two different voltage levels that are used in different places. For example, Vpgm and Verase power supply voltage levels are connected to the gate of the selected cell and the substrate, respectively. So, in order to electrically switch the voltage level from the single output (Vout), any switch would have to be placed between the output 110, and Vpgm and Verase voltage nodes. Such switch control would demand other local boosting to transmit the pumped power without a threshold voltage loss from the switch. Thus, the approach of FIG. 1 would not be a reliable solution for more than one voltage generations.
Accordingly, there is a need in the industry for an improved way of making two or more voltage levels in a flash memory.